1. Field of the Invention
The invention relates to magnetometers, devices for measuring magnetic fields, and more particularly to optical magnetometers.
2. Description of Related Art
A magnetometer is an instrument designed to measure the magnetic field within a measurement volume. In 1832, Carl Friedrich Gauss invented the first of these instruments by suspending a bar magnet in air using a thin wire. As magnetometers have developed, they've seen a number of uses, including calibrating magnets and measuring the magnetization of an object, but one of their most valuable commercial uses lies in the measurement of the magnetic field of the Earth itself. Local changes or disturbances in the Earth's magnetic field can indicate buried deposits of commercially valuable metals or metal ores, making magnetometers extremely useful for mining exploration and for geological applications in general. Some magnetometers also find military applications in the detection of submarines and other submerged vessels and objects.
One of the more common types of magnetometer takes advantage of the behavior of atoms and their constituent subatomic particles when subjected to magnetic fields. Most atoms and subatomic particles have natural magnetic moments arising from a property called the “spin” of the particle. If an atom is subjected to a magnetic field whose direction is not aligned with the axis of its magnetic moment, it will wobble or precess at a frequency known as the Larmor frequency, much as a spinning top wobbles relative to vertical (i.e., relative to gravity) when its spin is disturbed by an outside force. In the late 1950s and early 1960s, it was determined that very sensitive measurements of a magnetic field could be made by measuring and tracking the Larmor frequency of alkali metal atoms in vapor form that were placed in the magnetic field. In this type of magnetometer, called a Bell-Bloom magnetometer after its inventors, beams of light are used to place the atoms in a suitable state for measurement and to read the Larmor frequency.
The first Bell-Bloom magnetometer was described in U.S. Pat. No. 3,257,608, which is incorporated by reference in its entirety. In practice, an alkali metal (rubidium in the earliest Bell-Bloom magnetometers, cesium in most modern implementations) is placed in a closed glass cell, usually along with a buffer gas like nitrogen. The cell is heated to maintain a certain vapor pressure of the alkali metal. Light is then introduced into the cell to illuminate the alkali vapor.
The electrons in any particular atom can occupy a number of distinct energy states or levels that are dictated by the principles of quantum mechanics. However, only in certain energy levels will the electrons interact with the light of a particular wavelength. Thus, light at specific wavelengths introduced into the cell is used to place the atoms in a predictable quantum state in a process called optical pumping. If a typical atom in one of its ground (i.e., unexcited) states is hit with a photon of light, it will absorb energy from the photon, transition to a higher energy level, and then decay to some other ground state, releasing a photon in the process. As a result of optical pumping, the electrons preferentially occupy energy levels that do not interact with the incoming photons of light. Thus, the photons can pass through the vapor unimpeded and the atomic vapor is said to be polarized.
Once the vapor has measureable polarization, the measurement of the magnetic field using the same or another light source can take place. As was described above, Bell-Bloom magnetometers measure a magnetic field by measuring the Larmor frequency of the precessing atoms and by tracking changes in that frequency due to changes in the magnetic field. In practice, that is done by quickly switching the light source between two optical wavelengths, one of which is resonant with an optical absorption line and the other of which is not, at a rate equal to the Larmor frequency. When the optically-pumped atoms in the cell are exposed to light being switched at the Larmor frequency as described above, magnetic resonance occurs. The vapor in this case is maximally polarized and the light absorption by the cell reaches a detectable minimum, meaning that more light is transmitted through the cell. Switching the optical frequency at any other frequency, however, does not create a magnetic resonance. Thus, the minimum absorption point, indicative of the Larmor frequency, can be tracked.
The light that is not absorbed by the cell passes through it and strikes a photodetector. The output from the photodetector, after passing through a number of filters and amplifiers, is used both to determine and track the Larmor frequency and as an input to the light source to modulate it in the Bell-Bloom configuration. Alternatively, as was described briefly above, the output from the photodetector may also be used to drive an inductor coil that applies a magnetic field to the cell in the so-called Mx or Mz configurations. Either the modulated light or the applied magnetic field keeps the atoms precessing within the cell.
There are a number of areas where the performance of the typical Bell-Bloom magnetometer can be improved. For example, atoms occupying different ground states precess at slightly different Larmor frequencies. Thus, the detected magnetic resonance is not actually a resonance, but rather a group of resonances that are wider than their spacing, each with its own Larmor frequency. That is, the measured Larmor frequency is a composite of the Larmor frequencies of the group of atomic ground states. When the light source is switched or modulated at the composite Larmor frequency, the detected composite magnetic resonance may momentarily have specific magnetic resonances enhanced, thus altering the populations of those energy states and affecting the Larmor frequency itself. This can lead to measurement errors.
Additionally, the way in which the light source is modulated or driven can reduce sensitivity or contribute to error. Simply put, no light source will respond immediately or perfectly to a change in input. FIG. 1, a schematic diagram of input and output waveforms, illustrates this phenomenon. If the input to a light source such as a laser is a square wave 10 that alternates between wavelengths λA and λB, the actual response (i.e., output) of the laser may be a waveform like waveform 12, which has a lower, rounded amplitude overall and may be significantly phase-shifted (i.e., delayed) compared with the input square wave 10.